The present invention relates to a light scanning device for optical printers or the like.
An optical printer was recently used as an output device of a computer and is replacing the line printer. There are various types of optical printers. A liquid crystal printer is one type of optical printer. Such a liquid crystal printer comprises a crystal shutter array and utilizes an electrophotographic process.
A light scanning device for the liquid crystal printer is described below with reference to the accompanying drawings.
FIG. 5 shows a light scanning device 30 applied to the liquid crystal printer. The light scanning device 30 is placed to confront a peripheral surface of a photosensitive drum 20. The light scanning device 30 comprises a rod-shaped light source 11 such as a fluorescent lamp which extends parallel to an rotational axis of the drum 20. Light is radiated from the light source 11 toward a liquid crystal shutter array 15. The liquid crystal shutter array 15 comprises a plurality of liquid crystal microshutters 12 arranged in a line extending parallel to the axis of the drum 20, each microshutter 12 corresponding to a picture element. The microshutter 12 is formed of a liquid crystal and crystal voltages applied to the liquid change the microshutter's light transmitting property. The microshutter changes between a state where the microshutter allows the light to pass therethrough and a state where the microshutter prevents the light from passing therethrough. The state where the microshutter allows the light to pass therethrough will be referred to as a state where "the microshutter is opened", and the state where the microshutter prevents the light to pass therethrough will be referred to as a state where "the microshutter is closed", hereinafter. The liquid crystal shutter array 15 receives image signals and controls the microshutters in accordance with the image signals so that the microshutters may be selectively opened to allow the light from the light source 11 to pass through to generate optical dot patterns. Each dot corresponds to one picture element. Each dot of the generated dot patterns is in a square shape with its side being 85 micrometers, for a liquid crystal printer having a resolution of 300 dpi to which the optical scanning device 30 is applied. The optical dot patterns generated by the liquid crystal shutter array 15 are then focused onto the peripheral surface of the photosensitive drum 20, with the use of an optical focusing means 34 such as a fiber lens array (SELFOC lens array) having a magnification of 1 to form picture element patterns on the peripheral surface of the drum. One line on the peripheral surface of the drum 20 extending parallel to the rotational axis and confronting the light scanning device 30 is exposed to the dot pattern light. The drum 20 is rotated at a predetermined amount (rotational angle). Another line on the peripheral surface of the drum next to the exposed line confronts the device 30 and is exposed to the dot pattern light. Thus, lines on the peripheral surface of the photosensitive drum 20 extending parallel to the rotational axis are one by one exposed to the dot pattern light, as the drum is rotated. The above-described exposure system is called a "line scanning exposure system".
In the line scanning exposure system, it is necessary to use an optical system 34 such as a focusing fiber lens array. Therefore, the optical scanning device 10 utilizing such a line scanning exposure system is expensive and has a large size.
On the other hand, without the optical system 34, the light radiated from the liquid crystal shutter array 15 has a certain amount of spread angle, so that each dot of the dot patterns obtained on the peripheral surface of the drum 20 has a large size because the liquid crystal shutter array has a certain amount of spread angle. As a result, the dots of the dot patterns overlap with one another and lower the quality of the obtained picture image. Alternatively, the liquid crystal shutter array 15 may be arranged in close contact with the peripheral surface of the photosensitive drum 20 without the optical system 34. However, the toner carried on the drum is attached to the shutter array and such arrangement lowers the image quality.